REPRODUCTION IN HUMANS
Introduction
Need for sexual reproduction?
For genetic variety, so that species become stronger, and same diseases are not passed down/affect generations to come.
Egg + Sperm --> Zygote
(Gamete + Gamete --> Zygote)
Main Steps in Sexual Reproduction:
1. Meiosis - Production of gametes (egg/sperm) in both parents
2. Fertilization - Fusion of two gametes to make zygote
3. Mitosis - Development of zygote into an embryo, then fetus, then human.
Male Reproductive System
Urethra
- A tube which passes from the bladder through the penis outside to outside the body.
- Both semen and urine pass out through this.
Testis
- There are two testes. Each testis produces sperm
- Also produces male sex hormones (e.g. testosterone). These hormones are responsible for development/maintenance of secondary sexual male characteristics
- Each testis receives blood from blood vessels in a spermatic cord
- A narrow, coiled tube (epididymis) stores inactive sperm for testis, before they enter sperm duct.
Sperm Duct
- Each sperm duct loops over a ureter and opens into the urethra.
- After sperm is released from the epididymis, it travels through this duct
Glands & Secretion
1. The prostate gland is at the base of the urinary bladder where the two sperm ducts join the urethra. Beneath this is the Cowper's gland.
2. The seminal vesicle opens into each sperm duct. It temporarily stores sperm before they are released through the urethra.
3. All 3 glands secrete a fluid which mixes with sperm to make semen. This fluid contains nutrients and enzymes to nourish sperm, and stimulate them to swim actively.
Penis - An erectile organ which enters the vagina during intercourse to deposit semen, containing sperm.
Scrotum - Sac containing testes.
SPERMATOGENISIS
Semiferous tubules
Sertoli cells (support/nourish immature sperm)
Germ cells (sperm [>4 days])
Nuclear material in mitosis cells:
- one cell remains a stem/germ cell
- one goes through meiosis to develop as sperm
*Cytoplasm of a cell moves downwards to make tail of sperm
HEAD
- About 2.5 nanometers wide.
- Contains large nucleus with small amount of cytoplasm (haploid chromosomes - 23)
- Acrosome is a vesicle containing enzymes which break down parts of egg membrane for sperm penetration during fertilization
MIDDLE PIECE
- Contains mitochondria
- Mitochondria provides sperm energy to swim towards the egg
TAIL
- Beating movement of tail (flagellum) allows sperm to swim towards egg
- Sperm is mobile
CREATION & MOVEMENT
OF SPERM
TESTIS
EPIDIDYMIS
SPERM DUCT
URETHRA
OUTSIDE
SEMINAL VESICLE
Figure 3. Diagram of sperm
Female Reproductive System
Ovaries
- Two ovaries produce oocytes (unfertilized eggs)
- Produce female hormones
- Help oocytes develop into mature ovums
- Secondary female characteristics
- Necessary for pregnancy
- Menstrual cycle regulation (estrogen & progesterone)
NOTE - On the basis of DNA, the hormone of either male/female hormone will stop producing, helping determine the gender of the fetus
Figure 4. Diagram of female reproductive system
Fallopian Tubes
- A tube arriving from each ovary to the uterus, where the ovums move from the ovaries to the uterus
- Fertilization happens here
- Narrow muscular tube
Uterus
- An inverted pear shaped muscular organ located between the bladder and the rectum.
- Hollow muscular organ
- 7.5 cm x 5 cm x 2.5 cm (thick), with thick glandular lining called endometrium
- Uterus is 4 to 5 times its size during pregnancy
- Peristalisis movement during labor
1. Perimetrium - Outer Layer
2. Myometrium - Muscular Layer
3. Endometrium - Contains glands, mucous lining, hormones
ENDOMETRIUM
- Where embryo implants
- Contains numerous uterine glands that open into uterine cavity (lymphatic space)
Cervix
- Lower portion of the uterus where it joins the vagina. The opening of the cervix allows menstrual blood to flow out into the vagina during menstruation
- firmer consistency than the body of uterus (made from fibrous connective tissue
Vagina
- Leads from the cervix to outside the body
- Also called birth canal
- Opening of the vagina is called 'vulva'
- Semen is deposited in vagina during sexual intercourse
- Mucous membrane and outer smooth muscle coat closely attached to vagina
OOGENESIS
- 40,000 oocytes in ovaries --> Start developing during puberty
- Hormones that stimulate ovulation are produced in the pituitary gland (Lutenizing/FSH)
- Eggs in dormant stage till puberty
Ovulation - Release of mature eggs from female ovary. Normally, in humans, only one egg is released at one time; occasionally, two or more erupt during menstrual cycle (TWINS/TRIPLETS - Fraternal)
*Egg is released between 14th-16th day of the 28 menstrual cycle
Anovulatory cycle - Menstrual cycle where egg is not released
MITOSIS
OOGONIUM
MEIOSIS
POLAR BODY
SECONDARY OVUM
SECONDARY OVUM
MATURES
FALLOPIAN TUBE
In parthenogenisis, polar body fertilizes the ovum.
Male Sex Hormones
Female Sex Hormones
GONODOTROPIN RELEASING HORMONE
- Produced by the hypothalamus
- Stimulates release of two male hormones (FSH & Lutenizing hormones)
FOLLICLE STIMULATING HORMONE (FSH)
- Produced by anterior pituitary gland
- Stimulates production of sperm in testes
LUTENIZING HORMONE
- Produced by anterior pituitary gland
- Causes production of testosterone
TESTOSTERONE
- Secondary sexual characteristics
- Stimulates production of sperm in testes
- Enlargement of genitals
- Thickened larynx - leads to deeper voice
- Body hair
ESTROGEN
- Produced by ovaries, the adrenal gland and by body fat (in small quantities)
- Helps retain calcium in bones
- Regulates balance of HDL, LDL (High Density & Low Density Lipoprotein - relates to cholesterol) & cholesterol
- Aids in the maintenance of blood sugar levels
- Emotional balance
PROGESTERONE
- Produced in large amounts during/after ovulations
- Helps prepare uterus for implantation of a fertilized egg
- Helps reduce body fat, aids in relaxation & reduction of anxiety
- Promotes hair growth
TESTOSTERONE
- Produced in small amounts by ovaries and adrenal glands
- Important role in health and well being. Helps protect against osteoporosis
- Affects mood, energy, body fat
Ovulation
- The release of a mature egg from ovary
- Occasionally, two or more eggs may erupt during the menstrual cycle
- Egg erupts between 14th-16th day of 28 day menstrual cycle
*Another chapter has been devoted to cover the menstrual cycle in detail. Refer to this in MYP BIOLOGY.
Parturition - Process of giving birth
Human embryogenesis - Process of cell division and cellular differentiation of the embryo that occurs during early stages of development.
Embryogenesis - First eight weeks of development.
Embryo -> Fetus
Gestation Period: 9 months / 38 weeks
DEVELOPMENT OF CHILD DURING GESTATION PERIOD
1. Fertilization - Fusion of male and female gametes
2. Cleavage - Zyogte divides into 2 cells through mitsosis (2 --> 4 --> 8)
3. Blastulation - Cell differentiates into outer and inner cell mass. Blastocyst forms
4. Implantation - Embryo implanted into uterus wall
5. Embryonic Disk - Another layer in blastocyst with two layers. One becomes amniotic cavity, and other the yolk (used until placenta is formed)
6. Gastrulation - The process of re-organizing two-layer embryo into a three layer one.
7. Neurulation - Development of neural tissue
8. Development of nervous system
9. Blood cell development
10. Organogenesis
11. Development of face and neck
12. Development of eye
13. Development of limbs
Figure 5. Gastrulation
Figure 5. Gastrulation separates the embryo according to endoderm, mesoderm and ectoderm. (Pinterest, 2018)
Placenta
- An organ that develops in uterus during pregnancy
- Provides oxygen and nutrients to baby, removes waste product from baby's blood
- Attaches to wall of uterus. Umbilical cord connects baby and placenta.
Amniotic Fluid
- Protective liquid contained by amniotic sac
- Cushion for growing fetus
- Facilitate exchange of nutrients, water and biochemical products between mother and fetus.
BLOOD OF MOTHER AND BABY DO NOT MIX. Exchange of materials through a semi-permeable membrane (diffusion)
